Not an expert, but my understanding is that the multiverse (at least, what we today associate as the multiverse) came about due to the many worlds interpretation of quantum mechanics. Basically, quantum physicists had an observation - particles were moving as though they were being pushed by an invisible wave, and they would pick a random position based on that wave when observed.

The most prevalent explanation for this behavior is the Copenhagen interpretation, which states that the particle is the invisible wave, and the wave collapses into a particle when it is measured. But another common interpretation is the many worlds interpretation, which states that the invisible wave is just a statistical probability of where the particle is. And the reason why the particle seems to pick a random point on the wave when observed is actually because the particle creates branching timelines, and we can only observe what happens in our own timelines. Hence, it seems random to us.

I speculate that the idea of multiple parallel timelines, each slightly different, was probably pretty popular with scifi writers, especially since it’s an easy way to portray “what if” scenarios in their stories, and so the concept became popular because of that

According to a few Google searches it was a physisict Hugh Everett in the late 50s. I am sure there were others in more meta and philosophical concepts in the past but I get the idea he is the known physicist to come up with it.

Disclaimer I didn’t read a lot but his name came up a few times and I assume he had a theory that could relate to physics at the time.

I still like that Flash was probably the first for pop culture, when weird time travel started happening in other shows (after deep into Flash recent series) I started joking it was just Barry messing up crap between the multiverse and time.

Tbf the very concept of the universe as we understand it is a pretty new thing that we don’t actually know as much about as you may think.

We have known for slightly longer than a century that the Andromeda Galaxy was an entire galaxy, IE, we have only known for a hundred years now that there are any galaxies besides our own.

So that’s basically why parallel universes is such a “new” literary concept, even if ideas of there being other planes of existence predate that concept.

In ancient Indian cosmology universes are created and destroyed in an infinite cycle

It was popularised recently (in the past 100 years) because of superhero comic authors not wanting to be bound by preexisting canon

Personally, the first time I experienced this.`

The issue with ideas is that multiple people might have them at the same time. Also, ideas were shared verbally for most of human history. Written records don’t go back super far, so we can’t always know where an idea came from

I’d guess it pretty closely follows the development of concepts that put words to spacial dimensions - 2 points make a line, 2 lines make a plane, multiple planes establish a volume, etc

If you think of time as a line and follow the same logic as spacial dimensions, you have the ‘line’ that represents reality as you’ve experienced it, but every event that has more than one potential outcome branches out from that point the same way the axes that make up length, width, and depth branch from one point. Instead of a 3-D space, we have… well, the multi-worlds theory.

Started it in which time line? There are an infinite number of answers.

There are ancient cultures that developed the idea that inside mirrors are mirrored universes.

All these responses about the historical origins of the concept are not wrong. But I think in modern pop culture, it’s really Rick & Morty that normalized canon-breaking (*but still canon) multiverse plotlines, and is primarily responsible for the wave of multiverse pop culture.

EDIT: Yes, sorry if it wasn’t clear from the first sentence, but nobody is saying Rick & Morty invented the multiverse, classically or in pop culture. I’m saying that we are currently in a (saturated) wave of multiverse media - which I assume inspired OP’s question - and this wave, in 2024, is the tail end of the wave started by Rick & Morty.

Are we sure the Avengers didn’t start all this?